Matte finishes on optical fibers and other glass articles

ABSTRACT

A mixture of buffered hydrofluoric acid (e.g., HF and NH 4  F) and a treating agent (e.g., acetic acid, phosphoric acid, hydrochloric acid, sulfuric acid or formic acid) is used to produce a matte finish on the pure silica cladding of an optical fiber, on a borosilicate glass tube and on a sodium lime glass plate. The matte finish may be used to increase the adhesion of other materials to the glass (e.g., of metal coatings to silica fibers) or to reduce glare.

CROSS REFERENCE TO RELATED APPLICATIONS

This application was concurrently filed with our application Ser. No.07/486350 entitled "Wet Chemical Etching Technique for Optical Fibers".

BACKGROUND OF THE INVENTION

This invention relates to techniques for forming a matte finish on aglass article such as an optical fiber or a glass plate.

In the manufacture of glass articles, such as plate glass, it is oftendesirable to reduce glare by forming a textured surface, known as amatte finish, on the article to scatter light and reduce specularreflections. The following patents are representative:

H. Niederprun et al., U.S. Pat. No. 4,055,458, discloses the mat (matte)etching of glass (e.g., plate glass) by means of liquids containinghydrofluoric acid and a soluble fluorine-containing wetting agent, e.g.,perfluoroalkane sulfonic acid quaternary ammonium salt, aperfluoroalkane carboxylic acid salt, an alkoxylation product of aperfluoroalkane sulfonamide, or the like.

J. W. Falls, U.S. Pat. No. 3,616,098, describes glare-reducing glassproduced by including an undissolved inorganic salt (e.g., potassiumbifluoride, sodium bifluoride or calcium phosphate) in a hydrofluoricacid bath saturated with ammonium bifluoride or other salt.

On the other hand, in optical fiber technology, in particular the silicafiber segment of that art related to hermetic packaging ofoptoelectronic devices, it is well-known to form a metal coating on theexterior silica surface (cladding) of the fiber in order to solder thefiber to the package. The prevalent prior art technique for depositingthe metal coating is sputtering. This approach, however, is not withoutits limitations: (1) The need for a sputtering machine is cumbersome andexpensive; (2) Most sputtering machines are designed to deposit metalonto planar (e.g., semiconductor wafer) surfaces. The cylindricalsurface of a fiber and pragmatic considerations have led prior artworkers to utilize two separate sputtering steps-the first to depositmetal onto essentially half the cylindrical surface and the second todeposit onto the other half. However, where the "halves" meet,undesirable seams are formed which decrease reliability; and (3) Thesputtering process requires a vacuum environment to reduce backgroundcontamination. This requirement inherently precludes having attached tothe fiber anything which would introduce contamination into the vacuumchamber. Thus, one would remove the plastic jacket which coats the fiberand, furthermore, would avoid attaching a silicon block mount or pigtailconnector, for example.

Therefore, a metal deposition process which can uniformly coat curvedsurfaces and which is not so contamination sensitive, such aselectroplating, might be more desirable were it not for the fact thatthe electroplating of metal onto glass fibers suffers from pooradhesion.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a matte finish isproduced on at least a portion of a glass article by subjecting thatportion to a mixture comprising a fluorine-based etchant and a treatingagent comprising an acid which by itself, does not significantly etchthe article. Illustratively the treating agent is selected from thegroup consisting of acetic acid, phosphoric acid, hydrochloric acid,sulfuric acid, and formic acid. Depending on the particular treatingagent chosen, matte finishes can be formed on a variety of glassmaterials (e.g., borosilicate glass, pure silica and sodium lime glass)and on a variety of articles (e.g., glass tubes, silica optical fibersand glass plates).

Applications of the invention range from the packaging of optoelectroniccomponents to the reduction of glare on glass plates and the like. Inthe former case, an illustrative embodiment of the invention involvesforming a matte finish on the portion of a fiber to be soldered orotherwise hermetically sealed through the wall of the package. Beforeactually sealing the fiber to the package a metal coating is deposited,preferably electroplated, onto the matte finish portion. The mattefinish enhances the adhesion of the coating and thus eliminates the needto rely on expensive, cumbersome and less uniform prior art sputteringtechniques.

Another embodiment of the invention, therefore, is a silica opticalfiber having a matte finish on at least a portion of an exterior silicasurface of the fiber. Yet another embodiment is such a fiber with ametal coating on the matte finish portion, and still a third embodimentis an optoelectronic package incorporating such fibers.

BRIEF DESCRIPTION OF THE DRAWING

The invention, together with its various features and advantages, can bereadily understood from the following more detailed description taken inconjunction with the accompanying drawing, in which:

FIG. 1 is a schematic cross-sectional view of an optoelectronic packagewith an optical fiber hermetically sealed in a sidewall of the package;and

FIG. 2 is a schematic cross-sectional view of a metalized optical fiberin accordance with one embodiment of the invention.

DETAILED DESCRIPTION

Turning now to FIG. 1, there is shown a hermetic package 10 for anoptoelectronic component 12 (e.g., a semiconductor laser or photodiode)mounted on a block or carrier 14. In order to allow light to communicatebetween the interior and exterior of the package an optical fiber 16 isinserted through a hole in the sidewall 18. The fiber 16 is aligned withthe component 12 by well-known means not shown, and the end of the fiberadjacent component 12 has a lens thereon to enhance coupling efficiency.Preferably, the lens is formed by an etching process as described in ourconcurrently filed application. A hermetic seal 20 is formed between thewall and the fiber, illustratively by soldering. In order to ensure thata good quality seal is formed the portion 22 of the fiber which extendsthrough the wall is metalized.

In accordance with one aspect of the invention, as depicted in FIG. 2,the metalization of the fiber is preceded by an etching process whichproduces a textured or matte finish 24 to enhance adhesion of the metalcoating 26. As a consequence, relatively uniform metal coatings may bedeposited by straight-forward, well-known electroplating (e.g., oflayers of Ni and Au), rather than by sputtering.

In general, the textured or matte finish is formed by subjecting thedesired portion of the fiber to a mixture comprising a fluorine-basedetchant and a treating agent comprising an acid which by itself does notsignificantly etch glass. In a preferred embodiment for use with silicafibers, the etchant comprises buffered hydrofluoric acid and thetreating agent comprises acetic acid, hydrochloric acid or sulfuricacid.

EXAMPLE I

Single mode and multimode optical fibers having pure silica claddingsand 125 μm outside diameters were subjected to a mixture comprising byvolume 25% of 3:1 BOE and 75% of the commercially available, electronicgrade treating agent described below. The BOE (buffered oxide etch)solution contained 3 parts of 40% NH₄ F to 1 part 49% HF by volume andthis is known as 3:1 BOE. In each case below the mixture was at atemperature of about 20° C. and the fiber was exposed to the mixture forabout 50 minutes.

When 99% acetic acid as used as the treating agent, the mixture produceda fine grain (≲1 μm) matte finish on the exterior surface (silicacladding) of the fibers. This mixture was very attractive because itetched the fiber uniformly, not selectively (i.e., the etch rate was notdependent on the doping concentration or dopant species across the fiberend face). Moreover, we have found that matte finishes are produced whenthe proportion of acetic acid to 3:1 BOE exceeds about 55%.

When 37% hydrochloric acid or 97% sulfuric acid was used as the treatingagent, both mixtures produced a fine grain (≈5 μm) orange peel finish onthe exterior surface (silica cladding) of the fiber. Note, the mixtureof BOE and sulfuric acid was exothermic, and the fiber was not broughtinto contact with the mixture until the latter had cooled to about 20°C. In addition, the hydrochloric acid treating agent generated aprecipitate (probably NH₄ F salt).

When, however, 70% nitric acid, 88% formic acid or 86% phosphoric acidwas used as the treating agent, the fiber remained smooth--no matte orotherwise textured finish was observed.

As noted earlier, the invention is applicable to glass articles otherthan optical fibers. Examples II and III, below, relate to glass tubesand glass plates, respectively.

EXAMPLE II

Borosilicate glass (e.g., Pyrex) tubes were subjected to a mixturecomprising by volume 25% of 3:1 BOE, as defined in Example I, and 75% ofa commercially available, electronic grade treating agent. Exposure wasat about 20° C. for about 50 minutes.

When 99% acetic acid was used as the treating agent, a fine grain (≲1μm) matte finish was produced on the exposed surfaces of the tube.

When 86% phosphoric acid was used as the treating agent, a diffuse,crystal-shaped matte finish with feature sizes of ≈5 μm was produced onthe exposed surfaces of the tubes.

When 88% formic acid was used as the treating agent, the portion of thetubes below the surface of the liquid mixture did not exhibit anyobservable matte or otherwise textured finish. However, a matte finishwas produced on the portion of the tube above the surface of themixture; this result indicates that the vapor phase of the mixtureproduced the matte finish.

When, however, 97% sulfuric acid, 70% nitric acid or 37% hydrochloricacid was used as the treating agent, the tubes remained smooth--no matteor otherwise textured finish was observed.

EXAMPLE III

A sodium lime glass plate was subjected to a mixture comprising byvolume 25% of 3:1 BOE, as defined in Example I, and 75% of commerciallyavailable, electronic grade 99% acetic acid. Exposure was at about 20°C. for about 50 minutes. A fine grain (≲1 μm) matte finish was producedon the surfaces of the plate.

In summary, it appears that acetic acid is preferred as a treating agentinasmuch as mixtures containing it produced a fine grain matte finish onall three types of glass articles and etching was essentially uniform(i.e., nonselective with respect to different species, e.g., dopants) indifferent regions of the article.

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments which can bedevised to represent application of the principles of the invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention. In particular, other proportions ofBOE to treating agent (e.g., a few percent BOE to as much as about 50%BOE) and other proportions of HF and NH₄ F to water (e.g., 3:1 to 7:1)are expected to be suitable. In general, however, as with acetic acid,it is expected that a relatively high proportion of the treating agentis required to produce the desired textured or matte finish. Likewise, arelatively broad range of etching times and temperatures can be usedeffectively.

We claim:
 1. A method of forming a textured or matte finish on at leasta portion of a silica cladding of an optical fiber by subjecting saidcladding portion to a mixture comprising a fluorine-based etchant and atreating agent, said agent comprising an acid which, by itself, does notsignificantly etch said portion and which is selected from the groupconsisting of acetic acid, hydrochloric acid and sulfuric acid.
 2. Themethod of claim 1 wherein said mixture comprises by volume about 25% of3:1 BOE and 75% of said treating agent.
 3. The method of claim 1 furtherincluding depositing a metal layer on said portion having a textured ormatte finish.
 4. The method of claim 3 wherein said metal layer isdeposited by electroplating.